Improvements to the SSMEs
for increased margin and durability began with a formal Phase
II program in 1983. Phase II focused on turbomachinery to extend
the time between high-pressure turbopump overhauls by reducing
the operating temperature in the high-pressure fuel turbopump
and by incorporating margin improvements to the HPFT rotor dynamics
(whirl), turbine blade and HPFT bearings. Phase II certification
was completed in 1985, and all the changes have been incorporated
into the SSMEs for the STS-26 mission.

In addition to the Phase II improvements, additional changes
in the SSME have been incorporated to further extend the engines'
margin and durability. The main changes were to the high-pressure
turbomachinery, main combustion chamber, hydraulic actuators and
high-pressure turbine discharge temperature sensors. Changes were
also made in the controller software to improve engine control.

Minor high-pressure turbomachinery design changes resulted in
margin improvements to the turbine blades, thereby extending the
operating life of the turbopumps. These changes included applying
surface texture to important parts of the fuel turbine blades
to improve the material properties in the pressure of hydrogen
and incorporating a damper into the high-pressure oxidizer turbine
blades to reduce vibration.

Main combustion chamber life has been increased by plating a
welded outlet manifold with nickel. Margin improvements have also
been made to five hydraulic actuators to preclude a loss in redundancy
on the launch pad. Improvements in quality have been incorporated
into the servo-component coil design along with modifications
to increase margin. To address a temperature sensor in-flight
anomaly, the sensor has been redesigned and extensively tested
without problems.

To certify the improvements to the SSMEs and demonstrate their
reliability through margin (or limit testing), an aggressive ground
test program was initiated in December 1986. From December 1986
to December 1987, 151 tests and 52,363 seconds of operation (equivalent
to 100 shuttle missions) were performed. The SSMEs have exceeded
300,000 seconds total test time, the equivalent of 615 space shuttle
missions. These hot-fire ground tests are performed at the single-engine
test stands at the NASA National Space Technology Laboratories
in Mississippi and at Rockwell International's Rocketdyne Division's
Santa Susana Field Laboratory in California.